Use python3 as interpreter instead of python

[mhucka]

Style guidances and our own experiences with Python portability are such that invocations of python should be changed to use python3.

[gemini-code-assist]

Code Review

This pull request updates various scripts, configuration files, and documentation to explicitly invoke python3 instead of python. The feedback recommends reverting the changes inside virtual environments back to using the canonical bin/python executable, as it is more portable and guaranteed to exist across different platforms and virtualenv implementations compared to the bin/python3 symlink.

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[gemini-code-assist]

Inside a virtual environment, the canonical and guaranteed executable is always bin/python. While bin/python3 is often created as a symlink, it is not strictly guaranteed by all virtualenv/venv implementations or platforms. Since the virtual environment already isolates the Python interpreter to the correct version, using bin/python is safer and avoids potential issues with missing symlinks.

Suggested change

	"${tmp_dir}/${PYTHON_VERSION}/bin/python3" -m pip install --quiet --upgrade pip
	"${tmp_dir}/${PYTHON_VERSION}/bin/python" -m pip install --quiet --upgrade pip

[gemini-code-assist]

As with the pip installation step, using the canonical bin/python executable inside the virtual environment is safer and more portable than relying on the bin/python3 symlink.

Suggested change

	"${tmp_dir}/${PYTHON_VERSION}/bin/python3" -c "import openfermion; print(openfermion.FermionOperator((1, 1)))"
	"${tmp_dir}/${PYTHON_VERSION}/bin/python3" -c "import openfermion; print(openfermion.QubitOperator((1, 'X')))"
	"${tmp_dir}/${PYTHON_VERSION}/bin/python" -c "import openfermion; print(openfermion.FermionOperator((1, 1)))"
	"${tmp_dir}/${PYTHON_VERSION}/bin/python" -c "import openfermion; print(openfermion.QubitOperator((1, 'X')))"